Ash Golem

Fire Absorption. Whenever the golem is subjected to Fire damage, it regains a number of Hit Points equal to the Fire damage dealt.

Immutable Form. The golem can’t shape-shift.

Magic Resistance. The golem has Advantage on saving throws against spells and other magical effects.

Heated Form. A creature who touches the golem or hits it with a melee attack while within 5 feet of the golem takes 2 (1d4) Fire damage. In addition, nonmagical ammunition is destroyed immediately after hitting the golem and dealing any damage. Any nonmagical weapon takes a cumulative −1 penalty to attack rolls immediately after dealing damage to the golem and coming into contact with it. The weapon is destroyed if the penalty reaches −5. The penalty can be removed by casting the Mending spell on the weapon.

Multiattack. The golem makes two Slam attacks.

Slam. Melee Attack Roll: +7, reach 5 ft. Hit: 13 (3d6 + 3) Bludgeoning damage plus 2 (1d4) Fire damage. 

Fireball (1/Day). The golem casts the Fireball spell at 8th level, requiring no spell components and using Constitution as the spellcasting ability (spell save DC 17).

Entrap  (3/Day). Strength Saving Throw: DC 15, one Large or smaller creature the golem hit with a Slam attack on its turn. Failure: The target has the Grappled condition (escape DC 15), and the target takes 5 (1d10) Fire damage at the start of each of its turns while Grappled. The golem can only make one Slam attack while it has a creature Grappled.